I would need help to make a simple custom dimension with 3 custom biomes.

I really don't get how to do what I would like. I just would like to make a custom dimension named "pestifera''. Pestifera has to contain 3 custom biomes (very similar to those in the overworld). 1) "pestiferaforet" : a vanilla forest biome 2) "pestiferaneige" : a vanilla snowy plain biome 3) "champipestifera" : a muschroom plain biome (based on the vanilla muschroom island biome) With my current settings I juste have snowy places and plains/forests, but the muschroom biome even if recognized by the game, doesn't appear. Please help me 🥺